Capacitive potentiometer rebalancing system



ay 14, 1988 c. F WOLFENDALE 3,383,669

CAPACITIVE POTENTIOMETER REBALANCING SYSTEM Filed March 5, 1966 a Sheets-Sheet 1 IN R C Fuse, flauem\ s fificinunuc Y m; MA

ATTORNEY y 1968 c. F. WOLFENDALE 3,383,669

CAPACITIVE POTENTIOMETER REBALANCING SYSTEM Filed March 5, 1966 3 Sheets-Sheei 2 INPUT ATTORNEY y 968 c F. WOLFENDALE 3,383,669

CAPACITIVE POTENTIOMETER REBALANCING SYSTEM 3 Sheets-Sheet 5 Filed March 5, 1966 CELEB Benn -11w LNG TOR OLFIN DfiI-E BY AWM ATTORNEY United States Patent Ofifice 3,383,669 Fatented May 14, 1968 3,383,669 CAPACITIVE POTENTIOMETER REBALANCING SYSTEM Caleb Frederick Wolfendale, Great Brick Hill, near Bletcliley, England, assignor t Sogenique (Electronics) Limited Filed Mar. 3, 1966, Ser. No. 531,613 Claims priority, application Great Britain, Sept. 27, 1960, 33,144/ 60 7 Claims. (Cl. 340-200) ABSTRACT OF THE DISCLOSURE The application discloses an automatically balancing bridge including a capacitive potentiometer comprising a line of discrete stator electrodes coupled electrostatically to a movable pick-off electrode, a manually settable reference potential divider, a servo responsive to the potential difference between the reference divider tap and the pick-off electrode, and a potential divider connected to give a Zero off-set capability.

This invention relates to electric position-determining and posltion-control apparatus. More specifically, the invention relates to such apparatus employing a positionsensitive means comprising two relatively movable members, one of which is part of a capacitative potentiometer, and the other a cooperating electrode. An apparatus of this kind is described in my copending application Ser. No. 844,605, filed Sept. 29, 1959, now Patent No. 3,071,- 758.

One form of apparatus described in that application comprises a capacitative potentiometer consisting of a line of discrete conductive elements, and a pick-up electrode; the pick-up electrode is insulated from the elements, but is capacitatively coupled thereto. Means are provided whereby relative movement can occur between the pickup electrode and the line of elements, and as this relative movement occurs, the pick-up electrode assumes a potential the value of which is related to the potentials on the adjacent elements. The present invention concerns improvements in apparatus of this general kind.

The invention consists broadly of a position responsive apparatus comprising a potentiometric device including a series of elements adapted to be energised from a potential source to establish a spatial field distribution, a pickoff means mounted for movement relative to said elements for deriving from said field a potential which is a function of the relative position of said means and said elements, reference means adapted to be energised from said source for establishing a point of reference potential, position indicating or control means responding to a signal derived from the difference in potential and said pickoff means, and means for modifying the effective rotation etween the pick-off signal and said reference potential to provide an off-set facility.

Other features and advantages of the invention will appear from the following descriptions of embodiments thereof, given by way of example only in conjunction with the accompanying drawings, in which:

FIGURE 1 is a simplified circuit diagram of a positionresponsive apparatus;

FIGURE 2 is a simplified circuit diagram of another position-responsive apparatus;

FIGURE 3 is a simplified circuit diagram of yet another positionresponsive apparatus;

FIGURE 4 is a simplified circuit diagram illustrating an additional feature which is applicable to apparatus according to the invention;

FIGURE 5 is a simplified circuit diagram illustrating another additional feature which is applicable to apparatus according to the invention;

FIGURE 6 is a simplified circuit diagram illustrating yet another additional feature which is applicable to apparatus according to the invention;

FIGURE 7 is a simplified circuit diagram illustrating yet another additional feature which is applicable to apparatus according to the invention;

FIGURE 8 is a simplified circuit diagram illustrating yet another additional feature which is applicable to apparatus according to the invention;

FIGURE 9 is a simplified circuit diagram illustrating yet another additional feature which is applicable to apparatus according to the invention;

FIGURE 10 is a simplified circuit diagram illustrating yet another additional feature which is applicable to apparatus according to the invention.

One form of the invention is shown in FIGURE 1, and is an apparatus which can be used to move an element to a predetermined position. In FIGURE 1, a capacitative potentiometer, the parts of which are enclosed within the dotted rectangle 20, comprises a series of conductive elements 21 and a relatively movable element 22; in the present case the elements 21 are stationary, the electrode 22 being movable. Electrical potentials which are applied to the elements are derived from an alternating current source 23, across which is connected also .a first inductive device 24. The device 24 has on it a number of taps connected respectively to the elements 21.

In the manner described above, as the movable electrode 22 moves along the elements 21 it will assume a potential, with respect to a reference point, of its position relative to the elements of the potentiometer.

It can be arranged that the reference point is so chosen that when the electrode 22 attains a predetermined position with respect to elements 21 the potential of the electrode 22 is that of the reference point; hence, the positioning of the movable electrode can be observed, or controlled, by the difference between the electrode and the reference point, and the electrode can be brought to the predetermined position by moving it until this potential difference is Zero.

One means of establishing the reference point is shown in FIGURE 1; a second inductive device 29 is fed from supply 23, and device has a variable tapping point 30 upon it. The potential difference between tap 30 and electrode 22 is passed to amplifier 32 through transformer 31; amplifier 32 can control a reversible motor 33 which will drive electrode 22, as indicated by line 35, in a correcting, null-seeking sense until electrode 22 is in the preetermined position.

In operation, tap 30 is set to a position which corresponds to the desired position of the movable electrode 22. If the electrode is not in the desired position, there will occur between tap 30 and electrode 22 a potential which is a function of that difference; this potential difference can be referred to as an error signal. This error signal will by its magnitude and its phase be characteristic both of the extent and direction of the distance of the movable electrode from the desired position.

An indicator 34 can be used for indicating the position of a manually movable element coupled to the pickoff electrode with respect to the setting of the tap 30. Alternatively motor 33 could be mechanically linked to the tap 30 to vary its position on the potentiometer 29, until the error signal falls to zero; the position of tap 30 can be shown On any suitable indicating device, and will then indicate the position of the manually movable member.

In order that the value of the reference potential corresponding to any position of the pick-up electrode 22 should be off-set to a selectable extent; the inductive element 29 is supplemented by a further such element 52, with a variable tap 53. A transformer, indicated as an auto-transformer consisting of an inductor S4 tapped at 55, is connected between taps 53 and 36; the tap 55 establishes the reference potential.

The voltage at the reference point will depend upon the settings of both the variable taps 53 and 30, but the relative effects of the two adjustments will depend upon the ratio of auto-transformer 54. The ratio may or may not be unity.

FIGURE 2 shows another arrangement by which zero offset can be obtained. As shown, the number of elements in the capacitative potentiometer is greater than the number of taps on the inductive device 24 from which potentials are impressed on the elements. Two switches 55 and 56 are used to select which of the elements have a difference of potential impressed upon them, and the extent of the offset. The outermost contacts of switch 55, at each end, are connected together, so as to form as many separate contact groups as there are taps on device 24. The contacts of switch 56 are connected to the potentiometer elements. The arrangements of FIGURES 1 and 2 both enable the reference potential to be changed by a lurality of external setting controls. Other switching arrangements of this kind can be used to modify the distribution of potential on elements 21, as to the number of elements energised, the value of the impressed potential and its phase in relation to the reference potential.

FIGURE 3 shows an arrangement in which the reference potentials are adjustable in such a way as to provide an offset which is equivalent to alteration of scale,

and to give zero-setting. In this case, the voltages applied to the elements of the capacitative potentiometer 20 are derived from tappings on the inductive device 24 but the ends of device 29 are connected to two adjustable taps 58, 59 on the inductive element 29. Hence it will be seen that by adjusting the relative position of these taps, the magnitude of the voltage across the device 24 can be varied, and in consequence the magnitudes of the respective voltages on the elements of the potentiometer. In addition, a stepup effect could be obtained, reversing the arrangement and connecting the element 29 to two taps on the device 24.

In FIGURE 4, a similar scaling effect is obtained by the use of an additional auto-transformer 60, which is connected across an adjustable portion of the inductive device 29. The tap 61 on the autotransformer then establishes the reference voltage. It will be understood that these methods of scaling can be combined, for example by means of switches, to afford different scale ranges, sensitivities, offsets, or the like, as desired.

In FIGURE the connections between the taps on the inductive device 24 and the elements 21 each include one of the six secondary windings 25 of a trans former 26, whose primary winding 27 is also fed from source 23.

As a result of this arrangement, relative potentials on the elements 21 will be determined by the vector sum of the voltages at the taps on device 24 and the voltages appearing on the individual secondary windings 25. By appropriate selection of the taps and the magnitude of the voltages on the secondary windings, the nature of the potential gradient along the length of the series of elements 21 can be selected or adjusted to follow a wide range of characteristics. Thus, the gradient can be made to follow more closely a linear, or nonlinear, law and correction can be made for imperfections of the potentiometer or the positioning of the taps on device 24.

In FIGURE 5, modification of response of the apparatus is effected by transformer 26 associated with the potentiometer 20, but modification can also be effected by a similar transformer associated with the inductive device 29; an arrangement of this kind is shown in FIGURE 6. As shown in FIGURE 6, the capacitative potentiometer elements 21 are connected directly to the respective taps on device 24; the device is provided with a plurality of taps 37 and each is connected directly to an element 21. The inductive device 29 also has taps, at 39 and these taps are connected through secondary windings of transformer 26 to a series of selector switch studs 40. A further inductive element 41 can be connected to any two adjacent studs 40. The reference point is selected by choice of the studs 40 to which element 41 is connected, and by a variable tap 42 on element 41. The former gives a coarse control and the latter a fine one.

FIGURES 7 and 8 are arrangements in which the law of response of the apparatus follows a law that differs from linearity, and which may approximate a desired law. The taps 39 are spaced at predetermined, unequal intervals which will give a response law consisting of a series of straight line approximations to a curve. The arrangement of FIGURE 8 shows the use of a transformer 26 for modifying the voltages appearing at studs 40. An inductive element such as 41 can be used with the arrangements of FIGURES 7 and 8. FIGURE 8 is an example of another means by which the law of response can be changed; this consists in varying the voltage applied to the primary winding 27 of transformer 26. Thus, this voltage can be less than the voltage of source 23, by connecting the winding across part only of the inductive device 29, and as with the arrangement shown in FIGURE 8, the voltage may be adjusted with different settings of the tap 30. In FIGURE 8, this is achieved by connecting the primary winding of the transformer between a point on the element 29 and tap 30.

Guard rings or electrodes can be used with electrode 22, and FIGURE 9 is an example, where the guard electrodes are shown at 44 the guard electrodes are connected together and to the output of an amplifier 48 of unity gain. Amplifier 48 may form part of amplifier 32. By this means the potential of the guard electrodes can be brought substantially to that of electrode 22. Alternatively, the guard rings can be adjusted in potential with respect to the pick-up electrode 22, or to the elements 21, to give some desired effect; for example, the effective gap between the guard rings and the pick-up electrode can be varied in relation to the voltages on these elements.

In the arrangement shown in FIGURE 10, guard electrodes 44 are used also. These electrodes are maintained at earth potential and means are provided also whereby in operation the pick-up electrode 22 is also maintained at or near earth potential. For this purpose, included in series with the primary winding of transformer 31, and connected to earth is the secondary winding of a further transformer 50, the primary winding of which is fed from the output of amplifier 48. By suitable design of the transformers 31 and 50, in relation to the amplifier 48, it can thus be arranged that electrode 22 is at or near earth potential, without being connected to earth. The effect of the stray capacitance of the electrode, represented by C is thus reduced. It will be seen that the addition of this feed-back loop does not reduce the sensitivity of the bridge arrangement shown in FIGURE 10, because the winding of the transformer 50 is not effectively in series with the primary winding of the transformer 31. Both the arrangements shown in FIGURES 9 and 10 have the advantage of good sensitivity.

It is possible for the additional features described to be combined as appropriate. As an example, the connecting or modifying transformer 26 can be used in other than the arrangements in which it is shown in the drawing.

It will be appreciated also that the number of elements of the capacitative potentiometer, taps on devices 24, 29

or 39 and contacts on switches 55 and 56 can be different, either greater or less, from the numbers shown. Also, it may be convenient in some cases to combine elements such as 24 and 30 in a common inductive device. Again, two or more elements such as 20 can be combined, with the same or diiferent effective scales, to operate from a common element such as 30.

I claim:

1. A position responsive apparatus comprising:

a potentiometric device including a line of at least three conductive electrode plates, an inductive means for connection to a source of alternating potential, said inductive means having thereon a plurality of intermediate connecting points, electrical connections connecting said electrode plates to respective ones of said connecting points for establishing a progressively varying alternating potential field along the line of electrode plates, and pick-elf means movable along said line of electrode plates and capacitively coupled thereto for deriving from said potential :field a potential that is a smooth progressive function of the position of said pick-off means relative said electrode plates whereby to in terpolate between the potentials of said plates,

inductive means also connectable to said potential source to establish a point of reference potential and correlated with a position of said pick-off means,

output signal means responsive to the relative values of said reference potential and the potential of said pick-elf means for producing an output signal as a function of the relative position of said pick-off means along said line of plates and including offset means for generating a selectable potential difference and applying said selectable potential difference to the circuit between said pick-off electrode and said point, whereby to off-set to a selectable extent the magnitude of said reference potential from the magnitude otherwise corresponding to said position of said pick-off electrode.

2. An apparatus in accordance with claim 1 and comprising selective means for selectively establishing said field over a selected group only of the conductive plates in said line.

3. An apparatus in accordance with claim 2 wherein said selective means includes means for maintaining conductive plates of said line outside said group at potentials substantially equal to the nearest of the plates in said group.

4. An apparatus in accordance with claim 3 wherein said selective means comprises individual switch means included in said electrical connections for connecting said electrode plates to respective ones of said connecting points.

5. An apparatus in accordance with claim 1 wherein the last said inductive means comprises a potential divider means adapted to be energised from said source, said potential divider means having a plurality of points thereon at different potentials and connection contact-making means for selectively making connection to said points to establish said point of reference potential.

6. An apparatus in accordance with claim 5 wherein said oifset means includes a second potential divider means adapted to be energised from said source, said second potential divider means having also a plurality of intermediate connection points thereon at dilierent potential, further connection making means for selectively making connection to said points, and means for deriving a potential from the potential difference existing between the points selected by the respective connection means.

7. An apparatus in accordance with claim 6 wherein the last said means for deriving a potential includes means for combining with difierent effectiveness the potentials as selected by said contact-making means.

No references cited.

THOMAS B. HABECKER, Primary Examiner. 

